Recompression evaporator system and method



United States Patent U.S. Cl. 203--26 3 Claims ABSTRACT OF THE DISCLOSURE In a two stage evaporator, a first vapor recompression stage wherein the vapor released is mechanically compressed to a heat value in excess of that required for evaporation and, after utilization in said first stage, the excess non-condensed vapor is passed into indirect heat exchange with the liquid in a second non-recompression stage.

In a recompression evaporator, all of the power input to the compressor is converted to heat. This energy raises the pressure and temperature of the steam that is passed through the compressor. At 1,000 kw. input power to the compressor, 1,000 kw. becomes available in excess heat. Up to now this heat has been regarded as waste heat and has not been efiiciently utilized.

The present invention is especially useful in the evaporation of liquor which shows a fast rise in boiling point elevation versus liquor concentration. As will appear hereinafter, spent liquor in the kraft wood-pulping industry is one such liquor. At a 40% solids content of such liquor the boiling point rise is 8 F. considering that the economical pressure for recompression evaporators is around 5 p.s.i. pressure-increase from atmospheric pressure (p.s.i.g.), corresponding to a temperature increase of 16 F. from 212 F. to 228 F. for boiling water, an 8 F. boiling point elevation means that such liquor containing 40% solids will start boiling at a temperature of 220 F. The vapors at 5 p.s.i.g. pressure have a condensing temperature of 228 F. Instead of a 16 F. temperature differential, there will be a temperature differential of only 8 F. through the heating surface of the evaporator. For the incoming feed to the evaporator, the concentration can be assumed to be 18% solids. This liquor has a boiling point elevation of 2 F. which means that the boiling starts at 214 F. at atmospheric pressure giving a temperature differential of 14 F. when heated by vapors of 5 p.s.i.g. pressure. To utilize the higher temperature differential at the lower concentration, a recompression unit is usually divided in a number of sections connected in series on the liquor side and in parallel on the steam and vapor side.

The specific load, expressed in lbs. evaporation per sq. ft. an hour, drops down with increase in liquor concentration not only because of the reduction in temperature differential but also with the reduction in U-value, caused by the increase in liquor viscosity. When the solids concentration of the liquor rises from 18% to 40% in the example above, the U-value may go down 50%, which means a total drop in specific load of around 78% when the boiling point rise is taken into account. Thus there is then required a 3.5 times larger heating surface per lb. evaporation.

The later part of the evaporation, in which the solids concentration rises from 40% to 50%, which is a common requirement in the evaporation of liquor according to the present example, can not be economically accomplished in the recompression cycle. This is because with liquor having a 50% solids concentration, the boiling point elevation is around 13 F., leaving an efiicient 3,475,281 Patented Oct. 28, 1969 temperature differential of only 3 F. This requires a heating surface per lbs. evaporation per hour, which is at least 10 times greater than that required in the feed concentration section.

Briefly, in accordance with the invention, the liquor to be evaporated and concentrated is passed through a recompression evaporator stage. In this stage, the evaporation, sometimes called pre-evaporation herein, is accomplished at a reasonably low temperature differential, which is a condition for a low power requirement. The thus initially concentrated or pre-evaporated liquor is fed to a concentrator, either of single or multiple effect type, where it is further evaporated to the desired final concentration. The excess of heat developed from the compressor in the said initial stage is discharged in the form of vapors from such stage together with the condensate to a separator tank. Vapor from the separator tank is led to the concentrator to serve as the heating medium therefor.

This invention has among its objects the provision of a novel evaporation system combining recompression with a single or multiple stage evaporator, such system being adapted for handling liquor, the boiling point of which rises with an increase in solids concentration.

A further object of the invention is to provide a novel method which utilizes heat and power input in a recompression' evaporator in an optimum manner to produce a desired degree of concentration of the liquor with the smallest heating surface requirement.

The above and further objects and novel features of the invention will more fully appear from the following description when the same is read in connection with the accompanying drawing. It is to be understood, however, that the drawing is for the purpose of illustration only, and is not intended as a definition of the limits of the invention.

In the drawing:

FIG. 1 is a graph showing the boiling point rise of spent liquor with increase in concentration of the liquor; and

FIG. 2 is a schematic view of a preferred embodiment of a recompression evaporator system in accordance with the invention.

In FIG. 1 there is shown the relationship between the boiling point rise in F. upon an increase in the percent solid concentration of spent liquor from the Kraft pulping process. The same curve also applies to spent liquor from the neutral sulphite pulping process See TAPPI 40 No. 11:921.

Turning now to FIG. 2 of the drawing, the initial evaporator or pre-evaporator stage, here shown as a single effect evaporator 10, is the the indirect or surface evaporator type. Evaporator 10 has a main lower portion 11 having channel-forming plates or tubes (not shown) disposed therein, such plates or tubes providing two separated sets of channels. A conduit 15 is connected to a first one of said sets of channels to supply said channels with feed liquor to be evaporated. The housing of portion 11 of the evaporator 10 is provided with a port 12 adjacent its upper end for receiving heating vapor, such vapor flowing through the other, second set of channels in the portion 11 of the evaporator 10. Vapor and condensate from the heating vapor are discharged from the evporator 10 through a conduit 14.

Vapor from the evaporation of the feed liquor is discharged from the upper ends of the first channels of evaporator 10 into a vapor body 16 which forms the upper portion of the evaporator. The remaining, partially evaporated feed liquor is discharged from the upper ends of the first channels to be collected in the bottom of the vapor body 16 at a level below the upper ends of the channel-forming elements which project into the lower end of the vapor body. Such initially concentrated liquor is discharged from the lower end of the vapor body through a conduit 17, which leads the liquor to a concentrator 35, to be described.

Vapor is discharged from the vapor body 16 through a conduit 19 which extends to the inlet or suction port 22 of a compressor 24. A prime mover, such as an electric motor 25, drives the compressor, as shown. The vapor from the vapor body 16 is discharged under high pressure from port '26 of the compressor to a conduit 27 which is connected between such port and the heating vapor inlet port 12 of the evaporator 10.

The above-mentioned discharge conduit 14 connects the lower ends of the second set of channels to a separator tank 29. Condensate flowing through conduit 14 into tank 29 collects in the bottom of the tank and is discharged therefrom through a pipe 30 provided with a valve 31. In the embodiment shown the valve 31 is provided with a level control 32 which is responsive to the level of liquid in tank 29, so as to maintain the level of such liquid substantially constant.

Connected to the tank 29 adjacent the upper end thereof, and well above the surface of the liquid in the tank is a pipe 34 which conducts the vapor content of the discharge into a heating medium inlet port 36 of the concentrator 35. Such concentrator in the embodiment shown is of the indirect or heated surface type, having separated first and second channel sets in the lower portion 37 thereof. The above mentioned conduit 17 is connected to the lower end of the first set of channels of concentrator 35. Partially concentrated liquid from conduit 17 flows upwardly through such first set of channels to be evaporated or concentrated therein. Vapor given off by such concentrated liquid is discharged from the upper ends of such first channels and is collected in a vapor body 40, which forms the upper portion of the concentrator 35. Liquid transferred into the vapor body 40 is collected at the bottom of the vapor body 40, and is discharged therefrom as the desired product through a conduit 41.

The heating medium inlet port 36 of concentrator 35 is connected to the upper ends of the second set of channels in portion 37 of the concentrator. Condensate from the heating medium is discharged from the lower ends of the second channels of the concentrator through a conduit 39.

Vapor is discharged from vapor body 40 of the concentrator through a pipe 42 which is shown connected to an indirect heat exchanger or surface condenser 44 to supply the heating requirements of such condenser. Condensate from such vapor is discharged from heat exchanger 44 through a separate pipe 52. Process liquid such as cold water is fed to heat exchanger 44 through a pipe 45, such water being heated in heat exchanger 44 and then discharged therefrom through a pipe 46. The vapors remaining from the vapor heating medium which is fed to heat exchanger 44 are exhausted therefrom through a pipe 47 by a steam ejector 49. Such ejector receives steam by way of a pipe 50, and discharges such steam and the ejected vapor through a pipe 51, as shown.

Although only a single embodiment of the invention has been illustrated in the accompanying drawing and described in the foregoing specification, it is to be especially understood that various changes may be made therein without departing from the spirit and scope of the invention, as will now be apparent to those skilled in the art.

What is claimed is:

1. A method for evaporating liquor comprising, in a recompression evaporation system withdrawing hot vapors solely from evaporation of liquor in a first evaporator, compressing all such vapors mechanically and returning such compressed vapors with an excess of heat as indirect heating medium for the evaporation of further liquor introduced into said evaporator close to the boiling point thereof, drawing off the excess heat from said heating medium in the evaporator in the form of excess vapor resulting from said compression and introducing said excess vapor to serve as heating medium into an evaporative concentrator located outside of said recompression system, feeding said concentrator with liquor product from said recompression system and further concentrating said liquor product by passing the same in heat exchange relation with respect to said excess vapor.

2. A method as in claim 1 and withdrawing vapors resulting from the further concentration of said liquor product from said concentrator for use as heating medium in a heat exchanger separated from said evaporator system.

3. A method for evaporating liquor involving a recompression stage evaporator and a concentration stage evaporator which comprises in the recompression stage withdrawing hot vapors solely from evaporation of liquor of low concentration in said recompression evaporator, compressing all such vapors mechanically and returning such compressed vapors as indirect heating medium for the evaporation of further liquor introduced into said evaporator close to the boiling point thereof, drawing off the excess heat from said heating medium in the form of excess vapor resulting from said compression and introducing said excess vapor to serve as heating medium in said concentration stage evaporator outside of said recompression stage and introducing liquor from said evaporation stage having a high boiling point elevation into said concentration stage in indirect heat exchange relation with respect to said excess vapor.

References Cited UNITED STATES PATENTS 1,200,996 10/1916 Soderlund et al. 2,895,546 7/1959 Sadtler 159-24 3,075,578 1/1963 Sumiya 159-17 3,354,932 11/1967 Hesler et al. 159-17 3,362,457 1/ 1968 Chirico 159-17 2,769,489 11/ 1956 Eckstrom. 3,084,107 4/ 1963 McMahon 202-48 3,236,748 2/ 1966 'Pottharst 203-26 FOREIGN PATENTS 17,785 3/ 1881 Germany. 19,568 1914 Great Britain. 14,318 1962' Japan. 127,807 1919 Great Britain.

NORMAN YUDKOFF, Primary Examiner J. SOFER, Assistant Examiner U.S. p1. X.R. 1s9 17, 47; 202-174 

